Wide-angle zoom lens

ABSTRACT

A three-group zoom lens includes first, second, and third lens groups, of negative, positive, and positive refractive power, respectively. The second lens group includes a stop and the third lens group moves for focusing. When zooming from the wide-angle end to the telephoto end, the first and second lens groups become closer together and the second and third lens groups become farther apart. The zoom lens preferably satisfies specified conditions that ensure compactness, ease of manufacture, and favorable correction of aberrations. The zoom lens includes an aspheric lens component with lens surfaces defined by an aspheric lens equation that includes at least one non-zero coefficient of an even power of Y, and at least one non-zero coefficient of an odd power of Y, where Y is the distance of a point on the aspheric lens surface from the optical axis.

BACKGROUND OF THE INVENTION

Conventionally, zoom lenses for various cameras are formed, for example, of three-group construction and include, in order from the object side, a first lens group having negative refractive power, a second lens group having positive refractive power, and a third lens group. Zoom lenses with this construction have been widely used in order to produce a compact zoom lens with good correction of aberrations. For digital cameras and video cameras that have been widely used in recent years, as with zoom lenses for camera use in general, a small lens that enables high picture quality and low distortion is desired. Additionally, it is necessary to satisfy particular conditions due to the use of a solid state image pickup element, such as a CCD.

Recently, in these digital cameras and video cameras where a solid state image pickup element, such as a CCD, is used, the demand for a wider angle of view in the lens has become extremely strong. For example, there is a demand for a zoom lens in a thirty-five millimeter format camera to have a wide-angle focal length of approximately twenty-eight millimeters to twenty-four millimeters.

In a camera where a solid state image pickup device is used, it is possible to process an imaged picture into different pictures. This image processing, including image enlargement and cropping of an image taken at a wider angle, enables producing an image that simulates an image taken at the telephoto end to some extent. However, it is difficult to simulate a picture taken at a wide-angle from an image taken at the telephoto end. Therefore, it is necessary to optically obtain pictures at the wide-angle end.

Japanese Laid-Open Patent Application 2003-035868, Japanese Laid-Open Patent Publication 2001-296476, and Japanese said-Open Patent Publication 2000-284177 disclose zoom lenses designed for satisfying the requirements discussed above. The zoom lenses described in Japanese Laid-Open Patent Application 2003-035868 are mountable on a digital camera or a video camera where a solid state image pickup device, such as a CCD, is used. These zoom lenses have a three-group construction, wherein it is possible to zoom in and out within the range of focal lengths of twenty-six to eighty millimeters in terms of a thirty-five millimeter format camera. However, in the zoom lenses described in Japanese Laid-Open Patent Application 2003-035868, the first lens group from the object side, which is a lens group moved during zooming, is formed of three lens components that are lens elements so that it is difficult to satisfy the demands of compactness, which are currently strong for digital cameras and video cameras.

It has been accepted to use a minimum of three lens components that are lens elements for the first lens group in order to obtain acceptable optical performance at the wide-angle end of the zoom range of such zoom lenses. It has been considered difficult to make the first lens group more compact, for example, by including only two lens elements in the first lens group, with the optical performance being unsatisfactory, including unsatisfactory spherical aberration, distortion, lateral color, and image surface curvature (also known as field curvature).

BRIEF SUMMARY OF THE INVENTION

The present invention relates to a compact wide-angle zoom lens of simple construction with an object-side lens group including two lens components, which may be lens elements, and with satisfactory correction of aberrations, such as lateral color, spherical aberration, distortion, and image surface curvature, at the wide-angle end of the zoom range. The present invention further relates to a three-group wide-angle zoom lens particularly suited for mounting in a digital camera or video camera that uses a solid state image pickup element, such as a CCD, that is compact while providing a large wide-angle view.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from the detailed description given below and the accompanying drawings, which are given by way of illustration only and thus are not limitative of the present invention, wherein:

FIG. 1 shows cross-sectional views of the zoom lens according to Embodiment 1 at the wide-angle end (WIDE) and at the telephoto end (TELE);

FIGS. 2A-2D show the spherical aberration, astigmatism, distortion, and lateral color, respectively, of the zoom lens according to Embodiment 1 at the wide-angle end;

FIGS. 2E-2H show the spherical aberration, astigmatism, distortion, and lateral color, respectively, of the zoom lens according to Embodiment 1 at an intermediate position;

FIGS. 2I-2L show the spherical aberration, astigmatism, distortion, and lateral color, respectively, of the zoom lens according to Embodiment 1 at the telephoto end;

FIGS. 3A-3D show the spherical aberration, astigmatism, distortion, and lateral color, respectively, of the zoom lens according to Embodiment 2 at the wide-angle end;

FIGS. 3E-3H show the spherical aberration, astigmatism, distortion, and lateral color, respectively, of the zoom lens according to Embodiment 2 at an intermediate position; and

FIGS. 3I-3L show the spherical aberration, astigmatism, distortion, and lateral color, respectively, of the zoom lens according to Embodiment 2 at the telephoto end.

DETAILED DESCRIPTION

A general description of the three-group zoom lens of the present invention that pertains to the two disclosed embodiments of the invention will first be described with reference to FIG. 1 that shows Embodiment 1. In FIG. 1, lens elements are referenced by the letter L with a subscript denoting their order from the object side of the zoom lens along the optical axis X, from L₁ to L₆. Similarly, radii of curvature of the optical surfaces are referenced by the letter R with a subscript denoting their order from the object side of the zoom lens, from R₁ to R₁₄. The on-axis surface spacings along the optical axis X of various optical surfaces are referenced by the letter D with a subscript denoting their order from the object side of the zoom lens, from D₁ to D₁₃. In the same manner, the three groups are labeled G₁ to G₃ in order from the object side of the zoom lens and the lens elements belonging to each lens group are indicated by brackets adjacent the labels G₁ to G₃.

The term “lens group” is defined in terms of “lens elements” and “lens components” as explained herein. The term “lens element” is herein defined as a single transparent mass of refractive material having two opposed refracting surfaces that are oriented at least generally transverse to the optical axis of the zoom lens. The term “lens component” is herein defined as (a) a single lens element spaced so far from any adjacent lens element that the spacing cannot be neglected in computing the optical image forming properties of the lens elements or (b) two or more lens elements that have their adjacent lens surfaces either in full overall contact or overall so close together that the spacings between adjacent lens surfaces of the different lens elements are so small that the spacings can be neglected in computing the optical image forming properties of the two or more lens elements. Thus, some lens elements may also be lens components. Therefore, the terms “lens element” and “lens component” should not be taken as mutually exclusive terms. In fact, the terms may frequently be used to describe a single lens element in accordance with part (a) above of the definition of a “lens component.” The term “lens group” is herein defined as an assembly of one or more lens components in optical series and with no intervening lens components along an optical axis that during zooming is movable as a single unit relative to another lens component or other lens components. A “lens group” may also include one or more optical elements other than lens elements. For example, a lens group may include a stop that controls the amount of light that passes through the lens group.

The top portion of FIG. 1 shows the zoom lens at the wide-angle end of the zoom range and the bottom portion of FIG. 1 shows the zoom lens at the telephoto end of the zoom range. As shown in FIG. 1, the zoom lens is a three-group zoom lens that may include six lens elements and includes, arranged along the optical axis X in order from the object side, a first lens group G₁ of negative refractive power, a second lens group G₂ of positive refractive power, and a third lens group G₃ of positive refractive power. The second lens group G₂ includes a stop 2 that operates as an aperture stop to control the amount of light that passes through the zoom lens. In FIG. 1, a horizontal arrow before the label “Object side” points in one direction in order to indicate the object side of the zoom lens. The opposite side is the image side of the zoom lens. A filter unit or cover glass 1 is on the image side of the third lens group G₃. The filter unit may include a low-pass filter and/or an infrared cut-off filter for controlling the light flux to an image plane (not shown) where an image pickup element, such as a CCD, may be located.

During zooming from the wide-angle end to the telephoto end, as shown in FIG. 1, the first lens group G₁ and the second lens group G₂ both move to become closer together, and the second lens group G₂ and the third lens group G₃ become farther apart. In FIG. 1, a line that is concave toward the object side extends between the positions of the first lens group G₁ in the upper and lower portions of FIG. 1 in order to indicate the locus of points of movement of the first lens group G₁, as seen in the cross-sections that include the optical axis X, during zooming between the wide-angle end and the telephoto end. Similarly, a straight line between the positions of the second lens group G₂ in the upper and lower portions of FIG. 1 indicates the locus of points of movement of the second lens group G₂ toward the object side during zooming from the wide-angle end to the telephoto end. In the same manner, a straight line between the positions of the third lens group G₃ in the upper and lower portions of FIG. 1 indicates the locus of points of movement of the third lens group G₃, which in FIG. 1 is a vertical line in order to indicate that the third lens group G₃ remains stationary during zooming. However, the third lens group G₃ may also be movable during zooming. By this relative movement of the three lens groups G₁, G₂, and G₃ along the optical axis X, the focal length f of the entire zoom lens can be varied, and the light flux can be condensed efficiently on an image plane. Furthermore, when focusing is performed from infinity to nearby, the third lens group G₃ moves toward the object side.

The first lens group G₁ is formed of, in order from the object side, a first lens component that is a lens element L₁ having negative refractive power and a meniscus shape and a second lens component that is a lens element L₂ having positive refractive power and a meniscus shape.

Additionally, preferably in the zoom lens of the present invention, the first lens group G₁ includes at least one aspheric surface. The aspheric equation that defines the aspheric surface shape includes both even and odd aspheric coefficients that are non-zero, thereby providing both even and odd non-zero terms in the following equation for the aspheric shape of all the aspheric surfaces of the zoom lens: Z=[(C·Y ²)/{1+(1−K·C ² ·Y ²)^(½)}]+Σ(A _(i) ·|Y ^(i)|)  Equation (A) where

Z is the length (in mm) of a line drawn from a point on the aspheric lens surface at a distance Y from the optical axis to the tangential plane of the aspheric surface vertex,

C is the curvature (i.e., 1 divided by the radius of curvature, R) of the aspheric lens surface on the optical axis,

Y is the distance (in mm) from the optical axis,

K is the eccentricity, and

A_(i) is the ith aspheric coefficient, and the summation extends over i.

In the two disclosed embodiments of the present invention described below, for the aspheric surfaces of the first lens element L₁, aspheric coefficients A₃-A₁₀ are non-zero and all other aspheric coefficients of the first lens element L₁ are zero.

Conventionally, in the use of aspheric Equation (A) above, only even numbered aspheric coefficients A₄, A₆, A₈, and A₁₀ have been made non-zero in order to achieve the desired performance of the zoom lens. Using only even numbered aspheric coefficients A₄, A₆, A₈, and A₁₀ has been considered adequate to enable aspheric Equation (A) to be applied to satisfy the requirements for the lenses. Additionally, increasing the number of the non-zero aspheric terms has resulted in complicating optical design software and lens processing programming too much in view of computer performance capabilities.

However, in order to satisfy the demand for higher resolution lenses, the present invention employs improved computer performance of recent years and includes non-zero aspheric coefficients of odd-order terms. Because the number of parameters used to determine the aspheric shape increases, it becomes possible to determine the shape of the central region containing the optical axis of an aspheric lens surface and the peripheral region of the aspheric surface independently to some extent. In particular, by using a non-zero, third-order aspheric coefficient A₃ in order to provide a non-zero, third-order term, which is an odd-order term, in Equation (A), the rate of change of curvature near the optical axis can be increased.

In general, in a zoom lens that has a three-group construction, because an aspheric lens element arranged within the first lens group G₁ has the luminous flux spread out over the center portion and peripheral portion of the aspheric surface of the lens element, the lens element may be designed to refract the luminous flux in the peripheral portion so that image surface curvature and distortion aberration associated with the peripheral portion is well corrected. The configuration of the center portion of the aspheric lens surface, which contributes to spherical aberration, may be determined largely independently so that simultaneous excellent correction of spherical aberration, distortion, and image surface curvature can be achieved with both the center and peripheral portions by including non-zero, odd-order as well as even-order, aspheric coefficients in Equation (A), particularly at the wide-angle end where large field angles are being imaged.

Additionally, the greater the number of terms in the aspheric Equation (A) above, the greater the performance can be improved to some extent. However, the degree of difficulty of the design, processing and implementing the design becomes greater as the number of non-zero terms in the aspheric Equation (A) increases so that demands of better performance must be balanced against costs associated with the increasing difficulty. For example, simply adding one term of the third-order associated with a non-zero A₃, which is an odd-order term, to the fourth, sixth, eighth, and tenth order terms, which are even-order terms conventionally used in general, enables a reasonable improvement in correction of spherical aberration by its contribution to the shape of the center region of the aspheric surface.

Alternatively, in a zoom lens having a roughly similar construction to that relating to the present invention as described above and with the first lens group G₁ including an aspheric surface, the aspheric Equation (A) above that defines the aspheric surface shape may include non-zero, even-order terms of less than the sixteenth-order and at least one non-zero, even-order term of the sixteenth-order or higher instead of one or more non-zero, odd-order terms. This configuration may also result in improved performance as does using non-zero, odd-order terms. In other words, the configuration of the center portion of the aspheric surface that includes the optical axis and the configuration of the peripheral portion of the aspheric lens surface can be determined independently to some extent, and the configuration of the peripheral region can be made suitable for the correction of the spherical aberration due to the comparatively higher order term or terms. Simultaneously, the configuration of the center portion can be made suitable for the correction of the spherical aberration due to the comparatively lower order term or terms, enabling simultaneous excellent correction of spherical aberration, distortion, and image surface curvature, similar to the use of non-zero, odd-order terms in Equation (A) above.

Furthermore, the two alternatives described above may be used together. That is, Equation (A) above that defines the aspheric surface shape may include one or more non-zero, even-order coefficients, even of order sixteen or higher, and also one or more non-zero, odd-order coefficients as well.

Additionally, in the present invention, lens surfaces of other lens groups, that is, lens groups G₂ and G₃ may also be aspheric surfaces with their shapes given by Equation (A) above. Furthermore, Equation (A) that describes these aspheric surfaces may include non-zero, odd-order aspheric coefficients and/or non-zero aspheric coefficients of order sixteen or higher.

Additionally, in the zoom lens of the present invention, because (1) when zooming is performed from the wide-angle end to the telephoto end, the first lens group G₁ and the second lens group G₂ become closer together and the distance between the second lens group G₂ and the third lens group G₃ increases and (2) focusing is performed from the infinity end to a close focus by moving the third lens group G₃ toward the object side, the distance between the second lens group G₂ and the third lens group G₃ at the time of stowing the lens body in a retracted position can be reduced. Furthermore, a doublet is used in the second lens group G₂, which contributes to reducing the thickness of the second lens group G₂. Thus, compactness of the zoom lens in a retracted and stowed position can be increased by shortening the overall length of the zoom lens.

Additionally, preferably the zoom lens of the present invention satisfies the following Conditions (1)-(6): θ_(w)>41.0  Condition (1) ν_(d1)−ν_(d2)>23.4  Condition (2) ν_(dP)−ν_(dN)>25  Condition (3) 0.01<D _(A)<0.30  Condition (4) |R _(1P) −R _(2P)|/(R _(1P) +R _(2P))<0.3  Condition (5) 1.2<Fa/Fw<8.0  Condition (6) where

θ_(w) is the half-field angle of the entire zoom lens at the wide-angle end (the half-angle of view at the maximum image height at the wide-angle end),

ν_(d1) is the Abbe number at the d-line (587.6 nm) of the first lens element in order from the object side (i.e., lens element L₁ of the first lens group G₁),

ν_(d2) is the Abbe number at the d-line (587.6 nm) of the second lens element, in order from the object side (i.e., lens element L₂ of the first lens group G₁),

ν_(dP) is the Abbe number at the d-line (587.6 nm) of a biconvex lens element of the second lens group G₂,

ν_(dN) is the Abbe number at the d-line (587.6 nm) of a biconcave lens element of the second lens group G₂,

D_(A) is the distance on the optical axis between the image-side surface a cemented lens component and an adjacent object-side surface of a single-element lens component of the second lens group G₂,

R_(1P) is the radius of curvature on the optical axis of the image-side surface of the cemented lens component of the second lens group G₂,

R_(2P) is the radius of curvature on the optical axis of the object-side surface of the single-element lens component of the second lens group G₂,

Fa is the focal length of a single-element lens component of the second lens group G₂, and

Fw is the focal length of the entire zoom lens at the wide-angle end.

Condition (1) specifies a range of values at the wide-angle end of the zoom range for the wide-angle zoom lens of the present invention and is a condition that will be satisfied along with the other Conditions (2)-(6).

Satisfying Condition (2) in terms of the difference in Abbe numbers between the first and second lens elements of the first lens group G₁ helps control lateral color aberration that would otherwise be a problem at the wide-angle end.

Satisfying Condition (3) also helps to control lateral color at the wide-angle end, as well as helps to assure sufficient correction of axial chromatic aberration at the telephoto end.

By satisfying Condition (4), the length of the second lens group G₂ can be reduced, contributing to the compactness of the optical system.

By satisfying Condition (5), aberrations such as spherical aberration and coma can be corrected sufficiently even though the second lens group G₂ is made more compact.

By satisfying Condition (6), the quality of the manufactured lens components of the second lens group G₂ can be improved.

Accordingly, the wide-angle zoom lens of the present invention has the ability to correct various aberrations sufficiently even though the lens has a simple, six-lens-element construction and the overall length of the zoom lens in its stowed retracted position is short.

In Embodiments 1 and 2 of the invention disclosed below, all aspheric coefficients other than A₃-A₁₀ are zero. These two embodiments will now be individually described with further reference to the drawings.

Embodiment 1

In Embodiment 1, as shown in FIG. 1, the first lens group G₁ is formed of, in order from the object side, a first meniscus lens element L₁ of negative refractive power and a nearly plano-concave shape, with the nearly planar surface positioned on the object side, and a second lens element L₂ of positive refractive power and a meniscus shape with its object-side surface being convex. Both surfaces of lens element L₁ are aspheric surfaces with aspheric surface shapes expressed by Equation (A) above including both even-order and odd-order coefficients that are non-zero.

The second lens group G₂ is formed of, in order from the object side, the stop 2, a lens component doublet formed of, in order from the object side, a third lens element L₃ that is a biconvex lens element with its object-side surface having a greater curvature than its image-side surface and that is joined, as by being cemented, to a fourth lens element L₄ that is a biconcave lens element with its image-side surface having a greater curvature than its object-side surface and a fifth lens element L₅ of positive refractive power and a meniscus shape with its convex surface on the object side that forms a separate lens component of the second lens group G₂. Both surfaces of the fifth lens element L₅ are aspheric surfaces with the aspheric surface shapes expressed by Equation (A) above including only even-order coefficients that are non-zero.

The third lens group G₃ is formed of a sixth lens element L₆ of positive refractive power with its object-side surface being convex. Both surfaces of lens element L₆ are aspheric surfaces with the aspheric surface shapes expressed by Equation (A) above including both even and odd-order coefficients that are non-zero.

Embodiment 1 of the present invention is a three-group zoom lens that includes six lens elements with lens elements L₁, L₅, and L₆ having aspheric shapes defined as described above and that excellently corrects aberrations and enables forming a high resolution image. Additionally, the zoom lens of Embodiment 1 may be designed to have a reduced length in its retracted position.

Embodiment 1 includes the preferable feature of a lens component with aspheric surfaces with aspheric surface shapes expressed by Equation (A) above including both even and odd-order coefficients that are non-zero in the first lens group G₁. Additionally, Embodiment 1 includes the preferable feature of this aspheric lens component being a substantial distance from the stop 2. Because this arrangement allows for the luminous flux passing through the aspheric surfaces of this aspheric lens component to be well spread out between the center portion and the peripheral portion of the aspheric surfaces, this design is highly effective in simultaneously excellently correcting spherical aberration, distortion aberration, and image surface curvature.

Table 1 below lists numerical values of lens data for Embodiment 1. Table 1 lists the surface number #, in order from the object side, the radius of curvature R (in mm) of each surface on the optical axis, the on-axis surface spacing D (in mm) between surfaces, as well as the refractive index N_(d) and the Abbe number ν_(d) (at the d-line of 587.6 nm) of each optical element for Embodiment 1. Listed in the bottom portion of Table 1 are the focal length f and the f-number F_(NO) at the wide-angle and telephoto ends, and the maximum field angle 2ω at the wide-angle end and the telephoto end for Embodiment 1. TABLE 1 # R D N_(d) ν_(d)  1* ≈∞   1.50 1.75512 45.6  2* 4.8999 3.49  3 9.1536 2.50 1.92286 18.9  4 13.5419  D₄ (variable)  5 (stop) ∞ 0.40  6 5.7991 4.00 1.71300 53.8  7 −17.8297 0.70 1.84666 23.8  8 8.1732 0.10  9* 6.5615 1.88 1.68893 31.1 10* 14.8262 D₁₀ (variable) 11* 15.4501 2.00 1.58913 61.2 12* −25.0078 3.35 13 ∞ 1.00 1.51680 64.2 14 ∞ f = 3.8-13.8 mm F_(NO) = 2.5-5.2 2ω = 86.4°-26.8°

The lens surfaces with a * to the right of the surface number in Table 1 are aspheric lens surfaces, and the aspheric surface shapes of these lens elements are expressed by Equation (A) above.

Table 2 below lists the values of the constants K, and the coefficients A₃-A₁₀ used in Equation (A) above for each of the aspheric lens surfaces of Table 1. Aspheric coefficients that are not present in Table 2 are zero. An “E” in the data indicates that the number following the “E” is the exponent to the base 10. For example, “1.0E-2” represents the number 1.0×10⁻². TABLE 2 # K A₃ A₄ A₅ A₆ A₇ A₈ A₉ A₁₀ 1 −1.5605 3.5296E−4 1.2854E−3 −3.8582E−4 −1.7770E−5 2.6062E−5 −4.9102E−6   3.9405E−7 −1.2011E−8 2 −1.8708 −1.0750E−4   4.7249E−3 −8.8856E−4 −9.2004E−6 2.3140E−5   1.4295E−7 −6.4772E−7   5.3449E−8 9 −5.1774 0 2.4769E−3 0 −1.0601E−4 0   1.4997E−6 0 −3.3937E−7 10 −0.4707 0 2.2884E−3 0   7.3491E−5 0 −8.3428E−8 0 −2.4361E−7 11   7.4077E−1 1.1440E−3 5.4594E−4 −8.3878E−5   7.3456E−5 3.3985E−7 −7.9254E−7 −3.9032E−8   3.7778E−8 12 −1.4727E−1 2.7941E−3 1.2056E−4   2.8572E−4   3.2841E−6 −6.2274E−7     2.1747E−6   5.6464E−8 −3.0051E−9

In the zoom lens of Embodiment 1, the first lens group G₁ and the second lens group G₂ move during zooming. Therefore, the on-axis spacing D₄ between lens groups G₁ and G₂ and the on-axis spacing D₁₀ between lens groups G₂ and G₃ change with zooming. Table 3 below lists the values of the focal length f, the on-axis surface spacing D₄, and the on-axis surface spacing D₁₀ at the wide-angle end (f=3.8 mm), at an intermediate zoom position (f=8.8 mm), and at the telephoto end (f=13.8 mm). TABLE 3 f D₄ D₁₀ 3.8 16.93 4.30 8.8 5.79 12.20 13.8 2.78 20.00

The zoom lens of Embodiment 1 of the present invention satisfies Conditions (1)-(6) above as set forth in Table 4 below. TABLE 4 Condition No. Condition Values (1) θ_(w) > 41.0 43.2 (2) ν_(d1) − ν_(d2) > 23.4 26.7 (3) ν_(dP) − ν_(dN) > 25 30.0 (4) 0.01 < D_(A) < 0.30 0.10 (5) |R_(1P) − R_(2P)|/(R_(1P) + R_(2P)) < 0.3 0.11 (6) 1.2 < Fa/Fw < 8.0 4.13

FIGS. 2A-2D show the spherical aberration, astigmatism, distortion, and lateral color, respectively, of the zoom lens of Embodiment 1 at the wide-angle end. FIGS. 2E-2H show the spherical aberration, astigmatism, distortion, and lateral color, respectively, of the zoom lens of Embodiment 1 at an intermediate position, and FIGS. 2I-2L show the spherical aberration, astigmatism, distortion, and lateral color, respectively, of the zoom lens of Embodiment 1 at the telephoto end. In FIGS. 2A, 2E, and 2I, the spherical aberration is shown for the wavelengths 587.6 nm (the d-line), 656.3 nm (the C-line), and 435.8 nm (the g-line). In the remaining figures, ω is the half-field angle. In FIGS. 2B, 2F and 2J, the astigmatism is shown for the sagittal image surface S and the tangential image surface T. In FIGS. 2C, 2G and 2K, distortion is measured at 587.6 nm (the d-line). In FIGS. 2D, 2H and 2L, the lateral color is shown for the wavelengths 656.3 nm (the C-line) and 435.8 nm (the g-line) relative to 587.6 nm (the d-line). As is apparent from these figures, the various aberrations are favorably corrected over the entire range of zoom.

Embodiment 2

Embodiment 2 is very similar to Embodiment 1 and therefore is well shown by FIG. 1. Embodiment 2 differs from Embodiment 1 in its lens element configuration by different radii of curvature of lens surfaces, different aspheric coefficients of the aspheric lens surfaces, some different optical element surface spacings, and two different refractive materials.

Table 5 below lists numerical values of lens data for Embodiment 2. Table 5 lists the surface number #, in order from the object side, the radius of curvature R (in mm) of each surface on the optical axis, the on-axis surface spacing D (in mm) between surfaces, as well as the refractive index N_(d) and the Abbe number ν_(d) (at the d-line of 587.6 nm) of each optical element for Embodiment 2. Listed in the bottom portion of Table 5 are the focal length f and the f-number F_(NO) at the wide-angle and telephoto ends, and the maximum field angle 2ω at the wide-angle end and the telephoto end for Embodiment 2. TABLE 5 # R D N_(d) ν_(d)  1* 513.0309 1.50 1.75512 45.6  2* 4.8999 3.50  3 9.4145 2.43 1.92286 18.9  4 14.4962  D₄ (variable)  5 (stop) ∞ 0.40  6 5.8771 4.00 1.72916 54.7  7 −15.8019 0.64 1.84666 23.8  8 8.4670 0.10  9* 6.3737 2.44 1.68893 31.1 10* 12.8352 D₁₀ (variable) 11* 15.8473 2.00 1.50869 56.0 12* −20.5910 3.18 13 ∞ 1.00 1.51680 64.2 14 ∞ f = 3.8-13.8 mm F_(NO) = 2.5-5.1 2ω = 86.3°-26.8°

The lens surfaces with a * to the right of the surface number in Table 5 are aspheric lens surfaces, and the aspheric surface shapes of these lens elements are expressed by Equation (A) above.

Table 6 below lists the values of the constant K and the coefficients A₃-A₁₀ used in Equation (A) above for each of the aspheric lens surfaces of Table 5. Aspheric coefficients that are not present in Table 6 are zero. An “E” in the data indicates that the number following the “E” is the exponent to the base 10. For example, “1.0E-2” represents the number 1.0×10⁻². TABLE 6 # K A₃ A₄ A₅ A₆ A₇ A₈ A₉ A₁₀ 1 −1.5605 2.6931E−4 1.2997E−3 −3.9174E−4 −1.7815E−5 2.6181E−5 −4.9114E−6   3.9249E−7 −1.1917E−8 2 −1.8816 −1.8055E−4   4.7445E−3 −9.0074E−4 −9.2402E−6 2.3146E−5   1.4186E−7 −6.4828E−7   5.3255E−8 9 −4.4732 0 2.2056E−3 0 −1.0581E−4 0   1.5167E−6 0 −3.3931E−7 10 −0.4569 0 2.3791E−3 0   6.9893E−5 0 −9.4182E−8 0 −2.4363E−7 11   7.5087E−1 1.0995E−3 7.0219E−4 −6.8719E−5   7.3954E−5 3.8732E−7 −7.7511E−7 −3.6338E−8   3.8138E−8 12 −1.4792E−1 3.2421E−3 1.8728E−4   3.1202E−4   5.8212E−6 −5.0583E−7     2.1759E−6   5.5860E−8 −3.1127E−9

In the zoom lens of Embodiment 2, the first lens group G₁ and the second lens group G₂ move during zooming. Therefore, the on-axis spacing D₄ between lens groups G₁ and G₂ and the on-axis spacing D₁₀ between lens groups G₂ and G₃ change with zooming. Table 7 below lists the values of the focal length f, the on-axis surface spacing D₄, and the on-axis surface spacing D₁₀ at the wide-angle end (f=3.8 mm), at an intermediate zoom position (f =8.8 mm), and at the telephoto end (f=13.8 mm). TABLE 7 f D₄ D₁₀ 3.8 16.93 4.30 8.8 5.79 12.20 13.8 2.78 20.00

The zoom lens of Embodiment 2 of the present invention satisfies Conditions (1)-(6) above as set forth in Table 8 below. TABLE 8 Condition No. Condition Value (1) θ_(w) > 41.0 43.2 (2) ν_(d1) − ν_(d2) > 23.4 26.7 (3) ν_(dP) − ν_(dN) > 25 30.9 (4) 0.01 < D_(A) < 0.30 0.10 (5) |R_(1P) − R_(2P)|/(R_(1P) + R_(2P)) < 0.3 0.14 (6) 1.2 < Fa/Fw < 8.0 4.21

FIGS. 3A-3D show the spherical aberration, astigmatism, distortion, and lateral color, respectively, of the zoom lens of Embodiment 2 at the wide-angle end. FIGS. 3E-3H show the spherical aberration, astigmatism, distortion, and lateral color, respectively, of the zoom lens of Embodiment 2 at an intermediate position, and FIGS. 3I-3L show the spherical aberration, astigmatism, distortion, and lateral color, respectively, of the zoom lens of Embodiment 2 at the telephoto end. In FIGS. 3A, 3E, and 3I, the spherical aberration is shown for the wavelengths 587.6 nm (the d-line), 656.3 mn (the C-line), and 435.8 nm (the g-line). In the remaining figures, ω is the half-field angle. In FIGS. 3B, 3F and 3J, the astigmatism is shown for the sagittal image surface S and the tangential image surface T. In FIGS. 3C, 3G and 3K, distortion is measured at 587.6 nm (the d-line). In FIGS. 3D, 3H and 3L, the lateral color is shown for the wavelengths 656.3 nm (the C-line) and 435.8 nm (the g-line) relative to 587.6 nm (the d-line). As is apparent from these figures, the various aberrations are favorably corrected over the entire range of zoom.

The present invention is not limited to the aforementioned embodiments, as it will be obvious that various alternative implementations are possible. For instance, values such as the radius of curvature R of each of the lens elements, the shapes of the aspheric lens surfaces, the surface spacings D, as well as the refractive indices N_(d) and Abbe numbers ν_(d) of the lens elements are not limited to those indicated in each of the aforementioned embodiments, as other values can be adopted. Additionally, the present invention is not limited to a three-group zoom lens, and may include a two-group zoom lens or a zoom lens with four or more groups. Such variations are not to be regarded as a departure from the spirit and scope of the present invention. Rather, the scope of the present invention shall be defined as set forth in the following claims and their legal equivalents. All such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims. 

1. A zoom lens comprising, arranged on an optical axis in order from the object side as follows: a first lens group of negative refractive power; a second lens group of positive refractive power and that includes a stop for controlling the amount of light that passes through the zoom lens; and a third lens group of positive refractive power; wherein the first and the second lens groups are moved so that the first and second lens groups become closer together during zooming from the wide-angle end to the telephoto end; the second and third lens groups are moved relatively so that the second and third lens groups become farther apart during zooming from the wide-angle end to the telephoto end; the third lens group is moved toward the object side during focusing from infinity to a close focus; the first lens group includes, arranged on the optical axis in order from the object side, a first lens element of negative refractive power and a second lens element of positive refractive power, and at least one of said first lens element and said second lens element includes at least one aspheric lens surface; the shape of said at least one aspheric lens surface is given by an aspheric lens equation that includes at least one non-zero coefficient of an even power of Y, and at least one non-zero coefficient of an odd power of Y, where Y is the distance of a point on the aspheric lens surface from the optical axis; and the following conditions are satisfied: θ_(w)>41.0 ν_(d1)−ν_(d2)>23.4 where θ_(w) is the half-field angle of the entire zoom lens at the wide-angle end, ν_(d1) is the Abbe number at the d-line of said first lens element, and ν_(d2) is the Abbe number at the d-line of said second lens element.
 2. The zoom lens of claim 1, wherein the first lens group, the second lens group, and the third lens group are arranged along the optical axis without any intervening lens element.
 3. The zoom lens of claim 1, wherein the zoom lens includes only three lens groups.
 4. The zoom lens of claim 1, wherein the zoom lens includes only five lens components.
 5. The zoom lens of claim 4, wherein the zoom lens includes only six lens elements.
 6. The zoom lens of claim 2, wherein the zoom lens includes only five lens components.
 7. The zoom lens of claim 6, wherein the zoom lens includes only six lens elements.
 8. The zoom lens of claim 3, wherein the zoom lens includes only five lens components.
 9. The zoom lens of claim 8, wherein the zoom lens includes only six lens elements.
 10. The zoom lens of claim 3, wherein the zoom lens includes only six lens elements.
 11. The zoom lens of claim 1, wherein: said first lens element has a meniscus shape with its image-side surface being concave; said second lens element has a meniscus shape with its object-side surface being convex; the second lens group includes two lens components, an object-side lens component that includes only a biconvex lens element and a biconcave lens element and an image-side lens component that includes only one lens element, is of positive refractive power, and has a meniscus shape with its object-side surface being convex; the third lens group is formed of a single lens element of positive refractive power; each of said only one lens element and said single lens element includes at least one aspheric surface; and the following conditions are satisfied: ν_(dP)−ν_(dN)>25 0.01<D _(A)<0.30 |R _(1P) −R _(2P)|/(R _(1P) +R _(2P))<0.3 1.2<Fa/Fw<8.0 where ν_(dP) is the Abbe number at the d-line of said biconvex lens element, ν_(dN) is the Abbe number at the d-line of said biconcave lens element, D_(A) is the distance on the optical axis between said object-side lens component and said image-side lens component, R_(1P) is the radius of curvature on the optical axis of the image-side surface of said object-side lens component, R_(2P) is the radius of curvature on the optical axis of the object-side surface of said image-side lens component, Fa is the focal length of said image-side lens component, and Fw is the focal length of the entire zoom lens at the wide-angle end.
 12. The zoom lens of claim 11, wherein the first lens group, the second lens group, and the third lens group are arranged along the optical axis without any intervening lens element.
 13. The zoom lens of claim 11, wherein the zoom lens includes only three lens groups.
 14. The zoom lens of claim 11, wherein the zoom lens includes only five lens components.
 15. The zoom lens of claim 14, wherein the zoom lens includes only six lens elements.
 16. The zoom lens of claim 12, wherein the zoom lens includes only five lens components.
 17. The zoom lens of claim 16, wherein the zoom lens includes only six lens elements.
 18. The zoom lens of claim 13, wherein the zoom lens includes only five lens components.
 19. The zoom lens of claim 18, wherein the zoom lens includes only six lens elements.
 20. The zoom lens of claim 13, wherein the zoom lens includes only six lens elements. 